if i’m not mistaken, there is a sample with the fmod api that shows how to record from the Line-In and immediately play it to a stream callback.
You can then use the getspectrum function to get the spectrum.
Note that currently the streams delay is 100 ms and the minimum fmod latency is 50 (or 25?) ms.
Brett said in some reply that FSOUND_Stream_Setbuffersize was going to be added, and I hope it would allow streams with buffer sizes of 50 or less.
Then you could do a realtime spectrum analysis.